As the petroleum resource is decreasing, it is true that the crude oil feedstock becomes heavier and inferior. It is inevitable for the catalytic cracking, as the main treatment method for converting heavy oil to light fuel such gasoline and diesel oil, to treat heavy oil feedstock in the poorer quality in a larger quantity. Accordingly, much attention is put to the technology of catalytically cracking the heavy oil.
The effects on the catalytic cracking made by the crude oil feedstock becoming heavier and inferiorer are the decreased conversion and the increased coke yield. Therefore, many researches are done in different ways, including the molecular sieve modification, the catalyst production and the process design (Liu Tao, Zhang Zhongdong, Zhang Haitao, Sino-Global Energy. 2009, 14(1):71-77). However, most of the current catalyst design and industrial operation, the high yield of light oil and LPG is achieved by increasing the cracking reaction conversion, and therefore the coke yield is remarkably increased.
CN1436727A discloses a process for preparing a modified faujasite. According to that process, a faujasite, a phosphorus compound and an ammonium compound are firstly subjected to an exchange reaction, wherein the weight ratio of water to faujasite is 2-25, the pH is 2.0-6.5, the temperature is 10-150° C., and the exchange time is 0.1-4 hours. Then a rare earth solution is introduced to the exchange slurry. The reaction lasts for 1-60 minutes. After filtering and washing, the zeolite modified with phosphorus and rare earth is calcined at 250-800° C. under 1-100% steam for 0.1-3.5 hours to provide the final zeolite. The modified zeolite as prepared has a unit cell size of 2.440-2.465 nm, a sodium oxide content of 2.0-6.5 wt %, a phosphorus content of 0.01-3 wt %, and a rare earth oxide content of 0.1-15 wt % CN1624079A also proposes a similar process for modifying the molecular sieve, but the prepared molecular sieve has a relatively larger unit cell size.
CN1506161A discloses a rare earth ultrastable Y-type molecular sieve active component. This modified molecular sieve contains 8-25 wt % of rare earth oxide, 0.1-3.0 wt % of phosphorus and 0.3-2.5 wt % of sodium oxide, and has a crystallinity of 30-55% and a unit cell size of 2.455-2.472 nm. The molecular sieve is prepared with a NaY zeolite as starting material. The NaY zeolite is subjected to an exchange with rare earth and a first calcination to obtain a rare earth NaY that has experienced the first exchange and the first calcination. The rare earth NaY is then reacted with rare earth, a phosphorus-containing substance and an ammonium salt, and the resulting substance is subjected to a second calcination to obtain a Y-type zeolite modified with phosphorus and rare earth. This modified molecular sieve has a moderate coke yield. The molecular sieve as prepared has a relatively high rare earth content and a large unit cell size, which have a negative effect on the coke selectivity for the molecular sieve.
CN1317547A discloses a phosphorus and rare earth modified Y-type zeolite and a preparation process thereof. The molecular sieve is prepared with a NaY zeolite as starting material. The NaY zeolite is exchanged with a mixture of rare earth and an ammonium salt, and then subjected to a hydrothermal calcination, followed by reacting with a phosphorus compound and being subjected to a second calcination, wherein the RE2O3/Y-type zeolite weight ratio is 0.02-0.18, the ammonium salt/Y-type zeolite weight ratio is 0.1-1.0, and the P/Y-type zeolite weight ratio is 0.003-0.05. The calcination is conducted at a temperature of 250-750° C. under a steam condition of 5-100% for 0.2-3.5 hours.
CN101537366A discloses a modified molecular sieve that can have an improved coke formation property. The molecular sieve is prepared with a NaY zeolite as starting material and produced through two exchanges and two calcinations. The molecular sieve has a phosphorus content of 0.05-5.0 wt %, a RE2O3 content of 0.05-4.0 wt %, a unit cell size of 2.430-2.440 nm, and a crystallinity of 35-55%. The modified molecular sieve has a large hole volume of medium- and large-sized holes and a good stability. The modified molecular sieve has the advantages of reducing the catalyst's coke yield, simultaneously further improving the heavy oil cracking capability, further improving the total yield of liquid products, and particularly facilitating the improvement in the yield of light oil.
EP0421422 discloses a faujasite for hydrocracking. The faujasite absorbs an infrared in a frequency region of 3740±10 cm−1 in an absorption percentage A of at least 20% and absorbs an infrared in a frequency region of 3560±10 cm−1 in an absorption percentage B of at least 5%, the ratio of A/B being at least 2, has a specific surface area of at least 650 m2/g, has a framework SiO2/Al2O3 molar ratio of from 20 to 50, and has a unit cell size of 2.415 nm-2.450 nm.
CN1951814A discloses a modified Y-type zeolite, which has a SiO2/Al2O3 molar ratio of 7-30, a specific surface area of 700-900 m2/g, a unit cell size of 2.425-2.445 nm, a relative crystallinity of 80%, and a Na2O content of ≦0.25%. The secondary pores of a diameter ranging from 1.7 to 10 nm comprise more than 45% by volume of the total pores. The non-framework aluminum comprises more than 30% of the total aluminum. The modified Y-type zeolite has an infrared acid volumn of 0.15-0.55 mmol/g. The modified zeolite is prepared with NaY as starting material, and produced through an ammonium exchange, a hydrothermal treatment, a non-framework aluminum removal, a pore expansion, a second hydrothermal treatment and the like.
The above patent literatures disclose the Si/Al ratio of the Y-type zeolite is increased by hydrothermal dealumination and/or chemical dealumination, and the shrinkage of unit cell is achieved by the second hydrothermal calcination. However, during the course of the deep dealumination (SiO2/Al2O3 molar ratio≧15), the zeolite structure is often destroyed to decrease the zeolite crystallinity.